The study of somatomedin (sulfation factor), an active principle in the plasma whose secretion appears related to pituitary growth hormone, has been severely restricted by the in vitro cartilage explant system used for hormonal characterization. We have developed an in vitro chondrocyte monolayer preparation which can be used as a simplified assay for somatomedin and will allow the study of early hormone action on cartilage cells. The hormone responsivity of cartilage cells can be monitored by either thymidine H3 incorporation into DNA while the cells are proliferating or the incorporation of S35 into sulfated protein polysaccharide complexes (PPC) when the cells become contact inhibited. The object of this study is three fold. First, to characterize conditions necessary for maximal hormone expression for both proliferating and contact inhibited chondrocyte monolayers by varying the period of preincubation, the time in the growth curve of the monolayer culture, and the length of time the chondrocytes are exposed to the hormone and the radioactive nucleotides. The effects of insulin on these parameters will be investigated and compared to somatomedin. Concomitantly, somatomedin will be purified from human plasma, and the organ of origin of somatomedin will be determined by screening of specific tissues for the active principle. Care will be taken to inactivate known inhibitory factors present in plasma and particular attention will be paid to the pituitary gland, which, in preliminary experiments, is rich in somatomedin like activity. Subsequently, utilizing the purified somatomedin fractions, the early hormone effects on chondrocytes will be investigated. Care will be taken to delineate a hormone effect within the initial 15 to 30 minutes of exposure to hormone.